1. Technical Field of the Invention
This invention relates to an image forming apparatus and image forming method, and particularly to an image forming apparatus and image forming method for forming an image using an electrophotographic process.
2. Related Art
In an electrophotographic image forming apparatus, it is known that the characteristics of electrophotographic materials such as toner and photoconductive unit are changed by the variance in ambient environment such as temperature and humidity and the time period during which the apparatus is used, thus changing the density of a formed image. As a result, for example, halftone density of the image changes and a micro-point or line cannot be reproduced in the same size.
Thus, in many of the recent image forming apparatuses, an image quality adjustment mechanism is installed in order to prevent change in halftone density or secure reproducibility of a micro-point or line.
The image quality adjustment mechanism uses a method of maintaining the image quality by open-loop control, a method of maintaining the image quality by closed-loop control, a method combining these, or the like.
In the open-loop control, the environmental conditions, time period during which the apparatus is used and the like are monitored, and the process conditions such as quantity of exposure are changed by using a table provided in advance in the image forming apparatus, thereby maintaining the image quality.
On the other hand, in the closed-loop control, an image of a predetermined image patch is developed on a photoconductive unit in a state other than the time of image forming operation, and the patch density of the developed or transferred image is detected by a reflectance sensor, transmittance sensor or the like provided near the photoconductive unit or transfer target unit. On the basis of the detected density signal, the process conditions and the like are changed.
The stabilization of the gradation reproducibility and the reproducibility of a thin line or micro-point by such open-loop control or closed-loop control is broadly employed. Such control is generally called “image quality maintenance control”.
In a process in a typical electrophotographic apparatus, after a photoconductor such as a photoconductive unit is uniformly charged, light having intensity corresponding to the density of an image to be developed is cast onto the photoconductive unit, and the potential on the surface of the photoconductive unit is attenuated by optical attenuation, thus producing an electrostatic latent image. A laser diode or LED is used as means for casting light to the photoconductive unit, that is, exposure means.
In the image quality maintenance control, the quantity of exposure (exposure power or exposure energy density) of the laser diode, LED or the like is controlled in many cases.
Generally, if exposure is performed with a quantity of exposure that is twice to four times the half-potential exposure quantity of the photoconductive unit (the quantity of exposure required for attenuating the potential of a charged photoconductive unit to half), the potential of the photoconductive unit is attenuated almost completely and reaches a saturated attenuation state where the potential of the photoconductive unit hardly changes even if the quantity of exposure slightly varies. Therefore, if exposure is performed with the quantity of exposure that is twice to four times the half-potential exposure quantity, a stable potential of the photoconductive unit is provided in an area where pixels are not isolated points but are continuous (hereinafter referred to as solid area in some cases).
Utilizing this phenomenon, first, the charging potential of the photoconductive unit and the development bias are adjusted, and the difference between the development bias and the potent of the solid area (that is, development contrast) is adjusted, thereby deciding the density of the solid area.
Next, the gradation reproducibility is adjusted. For adjusting the gradation reproducibility, a method of controlling the exposure power of the laser diode, LED or the like, or a method of changing the type of halftone pattern is used. Other than these, there is a method of fine-tuning the charging potential of the photoconductive unit to adjust the gradation reproducibility.
As such image quality maintenance control, for example, JP-A-03-271763 discloses an image quality maintenance control method in which after a combination of grid potential of a charger and development bias potential is changed to adjust the maximum density of a solid area, the quantity of exposure is controlled on the basis of gradation correction data corresponding to that combination.
JP-A-06-83149 discloses an image quality maintenance method in which after the surface potential is controlled on the basis of a high-density pattern detection value, the quantity of exposure is controlled with a low-density pattern.
Also, JP-A-2006-11171 discloses a technique in which the number of image patches to be formed on an image carrier is reduced to one for image quality maintenance control. In this technique, two or more tables are provided in advance on the apparatus side, then the density of one image patch having an intermediate gradation level is detected, and adjustment of the development bias potential for adjustment of the density of a solid area is determined from the detected image patch density value and the tables. Next, the quantity of exposure is determined and adjusted from the same image patch density value and the tables provided in advance, and the halftone density and gradation reproducibility are adjusted.
In all of these techniques, it is assumed that intense exposure of the photoconductive unit is set with respect to the density of the solid area (saturated attenuation is done to set a stable area), and it can be said that these techniques are robust processes in terms of stabilization of the image. Therefore, image quality maintenance control can be realized by a relatively simple method.
However, not only higher image quality but also higher process speed is demanded of the recent image forming apparatuses.
A higher process speed can be realized by increasing the exposure power and securing exposure energy per unit area. However, high-output lasers or LEDs are costly, and particularly the high-output LEDs have a problem of heat generation or the like and they end up increasing in size. As for the laser diodes, the output is limited when they are arrayed in order to raise resolution.
Thus, a technique for forming an image of high image quality at a high speed while restraining the quantity of exposure (exposure power) is demanded. A technique for forming an image of high image quality with a small quantity of exposure, for example, a quantity of exposure equal to or less than twice the half-potential exposure quantity, instead of the intense exposure as in the conventional technique (the quantity of exposure set to be approximately twice to four times the half-potential exposure quantity of the photoconductive unit as described above), is necessary.
If the quantity of exposure (exposure power) is small, even when exposure is performed, the surface potential of the photoconductive unit is not sufficiently attenuated and it takes an intermediate potential state instead of a saturated potential state. Therefore, if the quantity of exposure changes, the potential of the solid area sensitively changes, too, and becomes unstable in a sense.
On the other hand, a method of realizing the adjustment of the development contrast potential by changing the quantity of exposure, utilizing the characteristic that the potential of the solid area sensitively changes, is known.
However, as a problem in setting such an intermediate potential, deterioration in the reproducibility of a thin line or micro-point, compared with the case of intense exposure, is considered, which is due to the sensitivity of the set potential to the quantity of exposure. This is for the following reasons.
In an ordinary exposure process, a scanning-type optical system is used in view of the speed, cost and the like. For example, a laser beam is caused to scan in the main scanning direction by using a polygon mirror, and a laser beam is caused to scan in the sub-scanning direction while a photoconductive unit is rotated. In the case where an LED line head is used, scanning in the sub-scanning direction is performed while a photoconductive unit is rotated, though beam scanning in the main scanning direction is not necessary. In such a scanning-type optical system, it is difficult to realize an ideal rectangular shape of exposure beam, and the beam has a shape that spreads to a certain extent such as Gaussian beam.
With such a spreading exposure beam shape, the exposure energy spreads and disperses in the direction of beam width. Therefore, particularly when a micro-point or thin line is to be printed, the peak value of the exposure energy is reduced and the potential of the photoconductive unit is not attenuated to a desired potential.
Meanwhile, if a solid area is exposed with a spreading exposure beam shape, the exposure energy of a substantially central part of the beam overlaps between neighboring pixels. Therefore, the potential of the photoconductive unit is largely attenuated, compared with the case of printing an isolated point such as micro-point or thin line. Thus, a large difference is generated between the potential of the photoconductive unit at the micro-point or thin line and the potential of the photoconductive unit in the solid area.
As a result, instability occurs such that if one tries to reproduce the thin line or micro-point sharply, the density of the solid area will become extremely high, whereas if one tries to adjust the density of the solid area to an appropriate level, the thin line or micro-point will be indistinct.
Moreover, if the reproduction of the thin line or micro-point is unstable, also the reproducibility of halftone and gradation tends to be more unstable than in the conventional case where the quantity of exposure is set at a large value. In the conventional image quality maintenance control method, it is difficult to provide sufficient stability.